The present invention relates to the detection of carryover particles in a furnace, such as a smelt bed boiler, and also to the use of information concerning detected carryover particles in the control of the furnace.
In general, carryover particles may be defined as "out-of-place" burning particles that are traveling in a furnace or boiler in a region well above the hearth of the furnace. More specifically, carryover particles in smelt bed recovery boilers may be defined as the mass of burning particles passing a horizontal plane at an upper level of the boiler, such as at the "bull nose" level within the boiler. Burning particles which encounter steam tubes in such a recovery boiler are quenched and form hard deposits on the tubing. These hard deposits are difficult to clean or remove through the use of typical steam cleaning mechanisms in such boilers.
A typical boiler is a liquor recovery unit used in mills for the manufacture of papermaking pulp. Such units typically require a substantial capital investment. In many cases the capacity of these boiler units limits the production of the pulp mill. A conventional liquor recovery unit is shown in FIG. 1 with a carryover particle detector system in accordance with the present invention. The recovery unit comprises a boiler 10 having a surrounding wall 12 through which water is carried for the purpose of steam generation. A typical modern unit of this type has a bottom area of about 50 square meters and a height of about 40 meters. Water tubes in the wall 12 and in the bottom of the incinerator or boiler are connected to a water drum, not shown, and, respectively, to a steam drum of a steam boiler. Through ports located about the circumference of the incinerator, normally at two or three different levels such as indicated by numbers 14, 16 and 18, combustion air is introduced into the boiler. Air is typically supplied into the boiler through these ports by large fans (not shown) with controlled dampers being used to adjust the air flow through these various ports. Schematically, the fans are represented in FIG. 1 as an air source 20 and some of the dampers are indicated as valves or dampers 22 and 24. A valve or damper controller 26, under the control of a process computer 28 and interface (not shown), control the operation of the various air supply dampers to control the flow of combustion air to the boiler. For example, to increase the rate of fuel combustion in the boiler, the amount of combustion air is typically increased. In addition, by supplying more air through selected ports than through other ports, an increase in the rate of consumption of fuel may be achieved in the regions of greater air supply to adjust the contour of a bed 30 at the bottom of the boiler.
Black liquor fuel enters the boiler through fuel nozzles 32, 34 as a coarse spray. Combustible organic constituents in the black liquor burn as the fuel droplets mix with air. Sodium sulfate in the fuel is chemically converted to sodium sulfide in the reducing zone of the boiler. The inorganic salts drop to the floor of the boiler to form a smelt bed 30, from which liquid is drained. The black liquor fuel is delivered from a fuel source 40 (from the pulp mill) and fed by conduits through respective valves 42, 44 to the nozzles 32 and 34 and hence to the combustion zone of the boiler. The process computer 28 and interface deliver suitable fuel control signals to the valve controller 26 for controlling the valves 42 and 44, and thus the supply of fuel.
It is desirable that combustion of substantially all of the black liquor fuel is carried out in the lower portion of the boiler 10, well below boiler steam tubes at an upper region of the boiler. However, in practice, dust particles formed in the hearth region of the boiler are carried along with flue gases upwardly through a restricted bull nose section 46 of the boiler. These particles in part adhere to the upper heat surfaces of the boiler. The dust typically contains sodium sulfate and sodium carbonate, but may also include other components to a varying extent. Under certain boiler or furnace conditions, such as resulting from disturbances in the air supply or perhaps due to a high bed volume in the boiler, uncombusted liquor fuel particles follow along with the upward gas flow. Such particles, as they burn, develop coatings on the heat surfaces which are removed only with great difficulty. Also, some of these particles burn as they contact the heat surfaces of the boiler and cause a sintering of other dust on the heat surfaces, again making the removal of these adhered particles very difficult. Thus, as hot gases from the combustion process entrain burning fuel particles and carry them upwardly, these particles may reach superheater tubes 47 and steam generator tubes 49 and may be deposited thereon. These tubes 47, 49 are conventionally used in such boilers for the generation of superheated steam for use in producing electrical power or for providing heat for other processes. As burning carryover particles impact these tubes, a buildup in the form of deposits occurs and tends to plug the passages between the tubes. Such a buildup reduces the heat transfer efficiency to the tubes and the boiler capacity. These deposits may eventually cause a shutdown of the boiler and also contribute to boiler tube corrosion.
For maintaining clean heat surfaces, including surfaces of the tubes 47 and 49, liquor recovery units are normally provided with a means for cleaning the heat surfaces. Such soot removal devices typically consist of pipes through which steam is injected while the pipes are being moved through the boiler. Even with these cleaning mechanisms, it is often necessary to stop the operation of the boiler for cleaning purposes. This often results in a loss of expensive pulp mill production time. In addition, these cleaning mechanisms are typically very effective at removing soft deposits on these tubes, but are much less efficient in removing the hard deposits formed by burning carryover particles.
The problems associated with the buildup of deposits from burning carryover particles on tubes of boilers have been recognized in the art. For example, U.S. Pat. No. 4,690,634 to Herngren, et al. describes an apparatus for counting burning carryover particles as they pass a detector. The count is used to indicate the occurrence of such carryover particles and/or in the control of the boiler operation. In the Herngren approach, a single optical detector is utilized which consists of a linear array of photo diodes (specifically 1024 diodes) arranged in rows. An optical lens is used to focus the diodes on a detection or focal plane, it being understood by the inventors that this detection plane is spaced only about two inches from the walls of the boiler. The resulting signal from the detector is amplified and compared to a threshold value which is used so that only signal peaks exceeding the threshold value are registered. The pulse width of the received signals is used in classifying the size of the particles. During a time interval, such as ten minutes, the device counts the number of detected carryover particle pulses within each particle size class, with the totals in the respective classes being converted to an analog current signal for delivery to a process computer.
The Herngren, et al. approach requires relatively complex and costly electronics to categorize carryover particles as to size. In addition, the use of a single detector positioned along one wall of a furnace, albeit with a linear array of photo diodes, permits in essence an examination of the boiler from one direction and, due to the limited depth of focal plane used in this approach, only a small region of the boiler interior is examined from this one direction. Consequently, localized disturbances in the smelt bed, which may result in the substantial production of carryover particles in a boiler region not within the single direction viewing utilized by Herngren, et al., may be missed.
Still another approach for monitoring the presence of carryover particles in a boiler is described in U.S. Pat. No. 4,814,868 to James. In the James approach, a single video camera imaging apparatus, such as of the type described in U.S. Pat. No. 4,539,588 to Ariessohn, et al., is disposed proximate to an upper portion of a recovery boiler for producing an analog video signal corresponding to the image of the interior of the boiler. The video signal is processed to eliminate noise and non-moving objects. A counter is used by James to count the occurrence of moving particles in the monitored region as a function of the relative magnitude of data points in the filtered signal and a predetermined threshold level. The particle count is incremented each time data points in the filtered signal exceed the threshold level. Such data points appear as a bright streak in the image and are caused by moving carryover particles. The imaging device of this patent is used to provide a video signal with a plurality of scan lines which are digitized and combined so as to discriminate between noise and burning particles. A display is used to display a visible image of the light emitting particles.
Through the use of a single camera, the James approach, like the approach of the Herngren patent, has a limited capacity to detect carryover particles other than along the wall which supports the camera. Due to the opaqueness of the environment in a typical boiler and the difficulty of detecting burning particles at significant distances (e.g. about three feet from the wall), the presence of carryover particles at distributed locations elsewhere in the boiler would tend to be overlooked by the Herngren, et al. and James devices.
In addition, the James approach is not understood to permit the discrimination between small carryover particles which are close to the camera and large carryover particles which are far away from the camera, as these particles appear to the camera to be of the same size.
Another system for detecting particles is disclosed in U.S. Pat. No. 3,830,969 to Hofstein. The Hofstein system utilizes a television camera for producing an image of a fluid sample with particulate matter therein. The image is processed to retain light points in the image which correspond to the moving particles. The resulting image is displayed on a CRT display or the like. The particulate matter is analyzed for characteristics such as movement, distribution, dimensions, number or concentration. There is no suggestion in this reference of operating such a system in the adverse environmental conditions present in a fuel fired furnace or boiler.
U.S. Pat. No. 4,737,844 to Kohola, et al. describes a system utilizing a video camera for obtaining a video signal which is digitized and filtered temporally and spacially. The digitized video signal is divided into signal subareas with picture elements belonging to the same subarea being combined into a continuous image area representing a certain signal level. The subareas are also combined into an integrated image with subsequent images being averaged to eliminate random disturbances. The averaged image is displayed on a display device. In an application described in this reference, the location, size and form of a flame front is determined from the image. This information on the flame front is used in the control of the combustion process. Although used in a furnace environment, this system is not directed toward the monitoring of carryover particles in a boiler.
In literature describing the device of the Ariessohn, et al. patent published in 1987, the smelt bed imaging system of such patent is described as providing clear, continuous images of the lower furnace char bed as well as of the deposit formation in the upper furnace. This literature does not set forth any details concerning the monitoring of deposit formations. Also, the device of the Ariessohn, et al. patent has been utilized in a commercially available product, called TIPS.TM., from Weyerhaeuser Company. This product relates to an electronic imaging device used in monitoring the temperature of the bed of furnaces, such as recovery boiler systems. An article by Mark J. Anderson, et al. entitled "Monitoring of Recovery Boiler Interiors Using Imaging Technology," published in April of 1989 by the Sensor and Simulation Production Division of Weyerhaeuser Company, describes this system in greater detail.
Although systems exist for use in monitoring the interior of recovery boilers and other furnaces, a need exists for an improved system for detecting carryover particles in the interior of such furnaces. This detected carryover particle information may then optionally be used in determining cleaning cycles for steam generation tubing and heat surfaces within the furnaces, for detecting abnormal conditions within the furnace which contribute to excessive carryover particle production, and for controlling the performance of the furnace so as to minimize the formation of such carryover particles.